Hydrodesulfurization is one of the fundamental processes of the refining and petrochemical industries. The removal of feed sulfur by conversion to hydrogen sulfide is typically achieved by reaction with hydrogen over non-noble metal sulfides, especially those of Co/Mo and Ni/Mo, at fairly severe temperatures and pressures to meet product quality specifications, or to supply a desulfurized stream to a subsequent sulfur sensitive process. The latter is a particularly important objective because some processes are carried out over catalysts which are extremely sensitive to poisoning by sulfur. This sulfur sensitivity is sometimes sufficiently acute as to require a substantially sulfur free feed. In other cases environmental considerations and mandates drive product quality specifications to very low sulfur levels.
There is a well established hierarchy in the ease of sulfur removal from the various organosulfur compounds common to refinery and petrochemical streams. Simple aliphatic, naphthenic, and aromatic mercaptans, sulfides, di- and polysulfides and the like surrender their sulfur more readily than the class of heterocyclic sulfur compounds comprised of thiophene and its higher homologs and analogs. Within the generic thiophenic class, desulfurization reactivity decreases with increasing molecular structure and complexity. While simple thiophenes represent the more labile sulfur types, the other extreme, sometimes referred to as "hard sulfur" or "refractory sulfur," is represented by the derivatives of dibenzothiophene, especially those mono- and di-substituted and condensed ring dibenzothiophenes bearing substituents on the carbons beta to the sulfur atom. These highly refractory sulfur heterocycles resist desulfurization as a consequence of steric inhibition precluding the requisite catalyst-substrate interaction. For this reason these materials survive traditional desulfurization and poison subsequent processes whose operability is dependent upon a sulfur sensitive catalyst. Destruction of these "hard sulfur" types can be accomplished under relatively severe process conditions, but this may prove to be economically undesirable owing to the onset of harmful side reactions leading to feed and/or product degradation. Also, the level of investment and operating costs required to drive the severe process conditions may be too great for the required sulfur specification.
A recent review (M. J. Girgis and B. C. Gates, Ind. Eng. Chem., 1991, 30, 2021) addresses the fate of various thiophenic types at reaction conditions employed industrially, e.g., 340-425.degree. C. (644-799.degree. F.), 825-2550 psig. For dibenzothiophenes the substitution of a methyl group into the 4- position or into the 4- and 6-positions decreases the desulfurization activity by an order of magnitude. These authors state, "These methyl-substituted dibenzothiophenes are now recognized as the organosulfur compounds that are most slowly converted in the HDS of heavy fossil fuels. One of the challenges for future technology is to find catalysts and processes to desulfurize them."
M. Houalla et al, J Catal., 61, 523 (1980) disclose activity debits of 1-10 orders of magnitude for similarly substituted dibenzothiophenes under similar hydrodesulfurization conditions. While the literature addresses methyl substituted dibenzothiophenes, it is apparent that substitution with alkyl substituents greater than methyl, e.g., 4,6-diethyldibenzothiophene, would intensify the refractory nature of these sulfur compounds. Condensed ring aromatic substituents incorporating the 3,4 and/or 6,7 carbons would exert a comparable negative influence. Similar results are described by Lamure-Meille et al, Applied Catalysis A: General, 131, 143, (1995) based on analogous substrates.
Mochida et al, Catalysis Today, 29, 185 (1996) address the deep desulfurization of diesel fuels from the perspective of process and catalyst designs aimed at the conversion of the refractory sulfur types, which "are hardly desulfurized in the conventional HDS process." These authors optimize their process to a product sulfur level of 0.016 wt. %, which reflects the inability of an idealized system to drive the conversion of the most resistant sulfur molecules to extinction. Vasudevan et al, Catalysis Reviews, 38, 161(1996) in a discussion of deep HDS catalysis report that while Pt and Ir catalysts were initially highly active on refractory sulfur species, both catalysts deactivated with time on oil.
Environmental and regulatory initiatives are also requiring lower levels of total aromatics in hydrocarbons and, more specifically, the multiring aromatics found in distillate fuels and heavier hydrocarbon products (i.e., lubes). The maximum allowable aromatics level for U.S. on-road diesel, CARB reference diesel and Swedish Class I diesel are 35, 10 and 5 vol. %, respectively. Further, the CARB and Swedish Class I diesel fuels allow no more than 1.4 and 0.02 vol. % polyaromatics, respectively.
Two types of process schemes are commonly employed to achieve substantial HDS/ASAT of distillate fuels and both are operated at relatively high pressures. One is a single stage process using Ni/Mo or Ni/W sulfide catalysts operating at pressures in excess of 800 psig. To achieve high levels of saturation pressures in excess of 2,000 psig are required. The other is a two stage process in which the feed is first processed over Co/Mo, Ni/Mo or Ni/W sulfide catalyst at moderate pressure to reduce heteroatom levels while little aromatics saturation is observed. After the first stage the product is stripped to remove H.sub.2 S, NH.sub.3 and light hydrocarbons. The first stage product is then reacted over a Group VIII metal hydrogenation catalyst at elevated pressure to achieve aromatics saturation. The two stage processes are typically operated between 575 and 1,000 psig.
In light of the above, there is a need for improved desulfurization/aromatic saturation process for treating feedstreams so that they can meet the ever stricter environmental regulations.